1. Field of the Invention
The present invention relates to a suspension system for supporting a power train in a vehicle from a vehicle body and more particularly, relates to a suspension system for a power train mounted transversely in a front and rear direction of the vehicle.
2. Description of the Related Art
When a power train assembled an engine and a transmission is mounted transversely in a front and rear direction of a vehicle, that is, crank shafts are mounted orthogonally in a front and rear direction of a vehicle, one example of a system for suspending the power train from the vehicle body is that mounts which support mainly a static load of the power train are mounted in a left and right of the power train which is on the vertical surface including a main axis of inertia in a rolling direction of power train (hereinafter, referred to as a roll main axis of inertia). Hereinafter, this system is referred to as a main axis of inertia mount system.
Here, a main axis of inertia is an axis, when a rigid body is rotated around an axis, that does not generate a moment which tends to change a direction of the rotational axis rotating with the rigid body in viewing from a coordinate axis. A roll main axis of inertia is inherent to a power train and passes the center of gravity of the power train.
Japanese Patent Laid-Open Publication No. 6-1151 discloses a main axis of inertia mount system, in which, in addition to the left and right mounts for supporting the static load of the power train, mounts absorbing a torque reaction force of the power train are provided in a front and rear direction of the power train and are called a front mount and a rear mount.
However, as described in the publication, it is well-known that when idling, influences to the vehicle vibration of vibration input to the front mount are big. As shown in FIG. 6, a vibration mode of a body frame is a two-section flexural vibration mode. This is because that in the vicinity of the power train, the closer to the extremity of the body frame is, the higher the sensitivity becomes. In FIG. 6, the solid line shows a frame of the body frame when an engine stops, while the short dash line shows a vibration mode of the body frame when idling.
Therefore, to increase the roll rigidity in the main axis of inertia mount system, it is advantageous for an idle vibration to make a spring constant in an up and down direction of the rear mount bigger than in an up and down direction of the front mount. This is because that if the spring constant of the front mount in the up and down direction make bigger, the influence to the vehicle vibration when idling becomes big.
Therefore, in the main axis of inertia mount system, it is considered to make the spring constant of the rear mount in the up and down direction bigger than the front mount in the up and down direction.
However, if the spring constant of the front mount and the rear mount are set as mentioned above, an elastic main axis in the roll direction (hereinafter, referred to as a roll elastic main axis) does not pass the center of gravity of the power train due to unbalancing of the spring in a suspension system. As a result, problems occur that a vehicle vibration when idling or a vehicle vibration accompanying with a cranking vibration generated when an engine is started become worse.
Here, an elastic main axis is an axis, when power is added along a particular axis in a spring device (suspension device), a direction of the power and a direction of the elastic displacement are coincided and does not generate an angular displacement. The elastic main axis only concerns with strength and placement of the spring and does not concern with mass or a center of gravity.